In phpEd, you do not actually "change the PHP version", but you "change the PHP version systax checking" from 5.3 to 5.2 or vice versa. This means, that you tell the IDE, in which version of PHP you want to write the code, so it could provide assistance.
The TechPlatform is a package installation with fixed configuration including with PHP 5.3. You cannot simply change the PHP version. In order to do so, you would need TechPlatform (or similar package) installed with PHP 5.2. Alternatively you can use XAMPP or Zend Server Community Edition - both free. But basically in any case you have to select, if you want to install PHP version 5.2 or 5.3.
It is also possible to install two servers, each with different PHP version 5.2 and 5.3. It is even possible to prepare a configuration, in which you could actually switch the versions. But in both cases, I know of none precompiled package and the manual configurations to achieve the desired result successfully is a bit tricky.